1. Field of the Invention
The present invention relates to a high-voltage pulse generating circuit of a simple arrangement for supplying a high-voltage pulse having an extremely short rise time and an extremely short pulse duration by releasing electromagnetic energy which has been stored in an inductor from a low-voltage DC power supply unit.
2. Description of the Related Art
There has recently been proposed a technology for generating a plasma by discharging a high-voltage pulse to perform deodorization, sterilization and also to decompose toxic gases. Generating such a plasma requires a high-voltage pulse generating circuit which is capable of generating a pulse having a high voltage and an extremely short pulse duration.
As shown in FIG. 15 of the accompanying drawings, a conventional high-voltage pulse generating circuit 100 comprises a capacitor charger 102, a capacitor 104, a switch 108, and a load 110 (see Japanese laid-open patent publication No. 2002-44965, for example).
The capacitor charger 102 generates a high DC voltage which is substantially equal to the peak value of a high-voltage pulse. The capacitor 104 is charged by the capacitor charger 102 to a voltage which is substantially equal to the high DC voltage generated by the capacitor charger 102. In order for the switch 108 to have a large withstand voltage, the switch 108 comprises a plurality of semiconductor devices 106 such as SI (Static Induction) thyristors or the like which are connected in series. The load 110 is supplied with a high-voltage pulse by high-speed switching operation of the switch 108 under the high DC voltage charged in the capacitor 104.
The switch 108 has a plurality of gate drive circuits 112 connected to the respective semiconductor devices 106 to turn on the semiconductor devices 106, and a plurality of balancing resistors 114 connected parallel to the respective semiconductor devices 106. The balancing resistors 114 serve to reduce any unbalances between the voltages applied across the respective semiconductor devices 106 due to impedance variations caused when the semiconductor devices 106 are rendered nonconductive.
Specifically, the high-voltage pulse generating circuit 100 has a multiple-series-connected circuit 116 of semiconductor devices 106 and balancing resistors 114 which are connected in series to the load 110.
FIG. 16 of the accompanying drawings shows a proposed high-voltage pulse generating circuit 118. In the proposed high-voltage pulse generating circuit 118, when a semiconductor switch 126 is turned on, a current flows from a DC power supply 120 (having a power supply voltage E) to a resistor 136 (having a resistance R) to the one-turn primary windings of respective maginetizable cores 128 to the semiconductor switch 126 to the DC power supply 120, the current having a magnitude represented substantially by E/R.
At this time, because of the maginetizable cores 128 operating as a transformer, the same current flows through the one-turn secondary windings of respective maginetizable cores 128 via the gates and cathodes of semiconductor devices 134. Therefore, all the semiconductor devices 134 are simultaneously turned on (see, for example, The Institute of Electrical Engineers of Japan, Plasma Science and Technology, Lecture No. PST-02-16).
The semiconductor devices 134 connected in series, and the semiconductor switch 126 are rendered conductive, a voltage which is substantially the same as the power supply voltage E is applied to an inductor 138. As a result, a current IL flowing through the inductor 138 increases linearly, storing electromagnetic energy in the inductor 138.
The current IL flowing through the inductor 138 increases until electromagnetic energy is stored up to a desired level in the inductor 138. When the semiconductor switch 126 is turned off, since the path of the current IL flowing through the inductor 138 is cut off, an induced voltage of opposite polarity is generated due to the stored electromagnetic energy in the inductor 138.
As a consequence, the diode 140 is rendered conductive, allowing a current to flow continuously from the inductor 138 to the semiconductor devices 134, the primary windings of the respective maginetizable cores 128 to the diode 140 to the inductor 138. At this time, a current of the same magnitude also flows through the secondary windings of the maginetizable cores 128.
Thus, the current flowing into the anodes of the semiconductor devices 134 flows entirely into the gates thereof, with no current flowing to the cathodes thereof. The current flows until the electric charges stored in the semiconductor devices 134 are discharged. Since no large voltage drop is caused in the current path and this state merely continues for an extremely short period of time, any reduction in the current IL flowing through the inductor 138 is small, and any reduction in the stored electromagnetic energy in the inductor 138 is also small.
As the electric charges stored in the semiconductor devices 134 are discharged, the semiconductor devices 134 are turned off, with a depletion layer being quickly developed therein. Since the inductor current is charged with a small electric capacity, the voltage between the anode and cathode of each of the semiconductor devices 134 rises sharply. Therefore, the voltage across the inductor 138 increases quickly, and the current IL flowing through the inductor 138 decreases quickly. Stated otherwise, the electromagnetic energy in the inductor 138 is shifted into capacitive electrostatic energy stored between the anode and cathode of each of the semiconductor devices 134. Since the voltage across the inductor 138 is also applied to a load 142 connected across the inductor 138, the electromagnetic energy in the inductor 138 and the capacitive electrostatic energy stored between the anode and cathode of each of the semiconductor devices 134 are consumed by the load 142 while the electromagnetic energy is being shifted into the electrostatic energy.
With the high-voltage pulse generating circuit 118, the DC power supply 120 may generate a low voltage and the semiconductor devices 134 may be turned on and off only by currents flowing through the secondary windings of the maginetizable cores 128. Consequently, the high-voltage pulse generating circuit 118 requires no gate drive circuits and is relatively simple.
However, the conventional high-voltage pulse generating circuit 100 shown in FIG. 15 has a complex circuit arrangement. A high voltage is applied to all the circuit components including the capacitor charger 102. The circuit components need to be insulated against each other, e.g., need to be spaced from each other by a large distance. Therefore, the conventional high-voltage pulse generating circuit 100 tends to be large in size and high in cost.
If only some of the series-connected semiconductor devices 106 are turned on due to malfunctions, then the remaining semiconductor devices 106 may be damaged by an overvoltage in excess of a rated voltage applied thereto. Accordingly, the operation of the conventional high-voltage pulse generating circuit 100 is not reliable.
Furthermore, for the conventional high-voltage pulse generating circuit 100 to generate a pulse which rises extremely sharply, e.g., at 10 kV/μsec or above, it is necessary that each of the semiconductor devices 106 be turned on quickly. Consequently, even if gate signals are applied to the semiconductor devices 106 at timings differing merely by 2 nsec or 3 nsec, or semiconductor devices 106 are turned on at timings differing merely by 2 nsec or 3 nsec, generated transient voltages are liable to be out of balance. The conventional high-voltage pulse generating circuit 100 thus suffers greater difficulty generating a pulse at several hundreds V/μsec than a series-connected array of semiconductor devices in an ordinary inverter.
With the proposed high-voltage pulse generating circuit 118 shown in FIG. 16, however, the DC power supply 120 may generate a low voltage, and a voltage in excess of the withstand voltage is never be applied to the semiconductor devices 134 even if some are turned off due to malfunctions. However, the timings of the turning off of the semiconductor devices 134 differ, making it highly difficult to prevent transient voltages from being brought out of balance when the semiconductor devices 134 are turned off quickly. Therefore, the proposed high-voltage pulse generating circuit 118 also suffers the same problems of series-connected semiconductor devices.
In the high-voltage pulse generating circuit 118, the maginetizable cores 128 are connected in series to the diode 140. As a consequence, inductances exist due to the physical distance in which the maginetizable cores are provided and also due to leakages between the finite primary and secondary windings. Because of these inductances, it takes time for the inductor current, which flows when the semiconductor switch 126 is turned off, to be commutated to the diode 140. As a result, the rate at which the gate current increases is suppressed, causing the semiconductor device 134 to remain conductive longer and the depletion region to spread (with the turn-off gain becoming one or more), which makes the high-voltage pulse generating circuit 118 unstable when the semiconductor devices 134 are turned off sharply.